1. Field of the Invention
The present invention relates to a developer and a development process for visualizing latent images in electrophotographic recording, electrostatic printing, magnetic recording, etc. More particularly, the invention relates to a developer which, in direct or indirect development processes for electrophotography, is positive-chargeable uniformly and strongly, and visualizes negative-electrostatic latent images to high quality images, and to a development process employing this developer.
2. Description of the Prior Art
A number of electrophotographic processes are known as disclosed in U.S. Pat. No. 2,297,691, Japanese Pat. Pub. No. 23910/67 (corresponding to U.S. Pat. No. 3,666,363), Japanese Pat. Pub. No. 24748/68 (corresponding to U.S. Pat. No. 4,071,361), and so forth. Electrophotographic processes generally comprise forming electrostatic latent images on photoconductor-containing photosensitive members by various methods, developing these latent images with developer powders (hereinafter referred to as toners), and if necessary, transferring toner images onto transfer media such as paper and fixing the resulting images with heat, pressure, or solvent vapors. When the process includes the transfer step, an additional step is necessary for removing the toner remaining on the photosensitive member after this transfer step.
A variety of development techniques are known including, for example, the magnetic brush process as disclosed in U.S. Pat. No. 2,874,063, the cascade process as disclosed in U.S. Pat. No. 2,618,552, the powder cloud process as disclosed in U.S. Pat. No. 2,221,776, the process employing a conductive magnetic toner as disclosed in U.S. Pat. No. 3,909,258, the process employing a high resistance magnetic toner as disclosed in Japanese Pat. Appl. Kokai No. 31,136/78, the process as disclosed in Japanese Pat. Appl. Kokai Nos. 42,121/79, 18,658/80, and 43,027/79, the fur brush process, the "touchdown" process, and the impression process.
The electronic printing process utilizing these techniques, as proposed in Japanese Pat. Pub. No. 14,342/67 and other literature, is a printing method in which an electrically charged toner is conducted onto a recording medium by utilizing electric fields and is fixed.
The electrostatic recording process comprises forming an electrostatic latent image on a dielectric layer, applying an electrically charged toner powder to adhere thereto, and fixed the toner image. Similarly the magnetic recording process comprises forming a magnetic latent image on a recording medium, developing the image with a toner containing a magnetic material, and transferring and fixing the toner image on a transfer medium.
Toners hitherto used for the above cited development processes are fine powders of natural or synthetic resins in which dyes or pigments are dispersed. An example of these toners is a fine powder of particle sizes 1-30 .mu.m prepared by pulverizing a dispersion of a colorant in a binder resin such as polystyrene and the like. Magnetic toners hitherto used are powders similar to the above but containing particles of a magnetic material such as magnetite and the like.
The development proceeses are roughly classified into the dry development process and the wet development process. The former is further divided into the process employing a two-component developer that comprises a toner and a carrier and the process employing a one-component developer that contains no carrier In the two-component developer, toner particles are usually glass beads, which are used in mixture with carrier particles such as powdered iron and the like.
Positive-charge regulators used in the toners for the dry development process include generally quaternary ammonium compounds and organic dyes, particularly basic dyes and salts thereof. Examples of positive-charge regulators generally used are benzyldimethyl-hexadecylammonium chloride, decyl-trimethylammonium chloride, nigrosine base, nigrosine, safranine .gamma., crystal violet and the like. In particular, nigrosine base and nigrosine are often used as positive-charge regulators. The positive-charge regulator is usually added to a thermo-plastic resin and dispersed by melting the mixture with heating. The resulting mass is finely pulverized, and if necessary, particle sizes of the powder are arranged in a suitable range. The powder thus obtained is used as a toner. However, dyes used as charge regulators are complicated in structure and have indefinite properties, thus being deficient in the constancy of product quality. Additionally, these dyes are liable to decompose or deteriorate under the influence of hot mixing, mechanical shock, friction, or variations in temperature and humidity conditions. Hence, the charge regulating ability of these dyes often lowers.
Accordingly, when a toner containing such a charge regulator is applied to a copying machine for development, the charge regulator may decompose or deteriorate as the number of copying times increases, causing the degradition of the toner.
Moreover, many positive charge regulators are hydrophilic, and hence on account of a poor dispersion of the hydrophilic dye (charge regulator) in a binder resin, bare particles of the dye will appear on surfaces of toner particles during crushing or grinding after melt-mixing of the dye and the binder resin. When such a toner is used under high humidity conditions, no good quality image will be obtained because of the hydrophilicity of the dye.
When such a conventional positive-charge regulator is used in a toner, considerable variation will arise in the electric charge generated the surface of toner particles between the toner particles, between the toner and carrier particle, or between the toner particle and the toner-carrying member, such as a sleeve employed. This tends to cause troubles such as fogging, toner scattering, and contamination with carrier particles. These troubles become remarkable when a large number of copies are made continuously. Thus such toners are substantially unsuited to copying machines.
Further, under high humidity conditions, many toners containing the conventional positive-charge regulator do not stand for use since the transferring efficiency of toner image lowers markedly. Even under normal temperature and humidity conditions, these toners, when stored for a long time, often deteriorate and become unusable, on account of the unstability of the positive-charge regulator.
Japanese Pat. Pub. No. 22,447/78 has proposed a method for preparing a positive-charge regulative developer, which comprises incorporating a powder of an aminosilane-treated metal oxide in a developer. The present inventors minutely investigated this method. That is, developers were prepared according to examples described in the specification of said patent, by using metal oxides, e.g. colloidal silica, alumina, titanium dioxide, zinc oxide, iron oxide, .gamma.-ferrite, and magnesium oxide, which had been treated with various aminosilane. However, any of these preparation experiments did not give a developer exhibiting satisfactory characteristics for practical use. These developers have proved to have several drawbacks.
That is, most of the developers cannot retain favorable characteristics for high fidelity development of latent images. Although exhibiting desirable functions at first, these developers lose initial characteristics during long term continuous survice, becoming unfit for use. That is, they cause fogging and when reproducing a linear pattern, they scatter and form spots in the neighborhood of the edges. Additionally they form lower density images. Other drawbacks of these developers are that, in development and transfer under high temperature and humidity environmental conditions, they result in lowered image density, the toner scattering around a linear pattern image, fogging, and toner-undeposited sites in image areas.
Most of these charge regulators are colored. Such being the case, these charge regulators are difficult to use in positive-chargeable color toners.
On the other hand, the following processes are known for the development by use of one-component nonmagnetic toners.
One development process employs a development system comprising a movable developer-carrying means for carrying a developer and supplying it to a latent-image-holding member, a developer-supplying means, and a movable coating means which serves to receive the developer from the developer-supplying means and to apply the developer onto the movable developer-carrying member. The movable coating means has a fiber brush for holding the developer on the surface, faces the movable developer-carrying means, and moves in the same direction at the facing position as does the movable developer-carrying means and at a higher speed than the movable developer-carrying means. In this process, the developer is uniformly applied with the movable coating means on the movable developer-carrying means, and the coating layer is allowed to approach to an electrostatic latent image area on the latent-image-holding member, thereby developing the latent image.
Another development process employs a development system comprising (i) a rotatable magnetic roller for forming a magnetic brush by absorbing a magnetic carrier which has been mixed with a one-component nonmagnetic toner for the purpose of charging particles of the toner, and (ii) a development roller for taking toner particles from the magnetic roller and for developing an electrostatic image on an electrostatic-image-holding member. The development is carried by keeping the gap between the electrostatic-image-holding member and the development roller at a value larger than the thickness of the toner layer held on the development layer.
Another development process is a method of developing an electrostatic image on an electrostatic-image-holding member by opposing a developer holding member, which holds a developer on the surface, to the electrostatic-image holding member, wherein the developer in a developer-storing means positioned under the developer-holding member is drawn up onto the developer-holding member while vibrating the developer present in the drawing-up portion to activate this developer, whereby a developer layer having a prescribed thickness is formed on the developer-holding member and served for the development.
However, in these process in which the development is conducted by carrying an insulating nonmagnetic toner on a toner-carrying member by the action of nonmagnetic force in the development section, electrostatic attractive force or physical adhesive force is dominant in the development region as the force to retain the nonmagnetic toner on the toner-carrying member. In this respect these processes involve various disadvantage as compared with the development process employing a conventional insulating magnetic toner wherein the toner is retained on a carrying member by the action of magnetic force and electrostatic attractive force. For instance, many toners fail to form a relatively thin, uniform layer on the carrying member. Further, for instance, a toner adheres to the non-image area, that is, so-called background fogging occurs, whereas the toner coating layer on the carrying member is relatively uniform. The amount of the toner adhering to the image area is deficient and consequently the image density is low, whereas the toner coating layer is thin and uniform. Whereas many toners can form a thin uniform coating layer, the formed image is low in fidelity and very poor in resolution. Many toners, during repeated use, result in the reduction of image density and the deterioration of image quality. Under various environmental conditions such as high temperature and humidity conditions and low temperature and humidity conditions, many toners result in the reduction of image density in some cases and fog in some other cases. One-component magnetic toners contain large amounts of a magnetic powder and are therefore expensive as compared with nonmagnetic toners. Additionally it is difficult to form bright color images in the development process employing one-component magnetic toners
In recent years various techniques and devices are developed relating to the stage of fixing toner images on paper or the like in image-forming instruments such as electrophotographic copying machines The most common fixing technique today is the so-called heat roll fixing technique wherein an image-receiving sheet bearing a toner is brought into contact with a heated roll to fix the toner image. However, when such a fixing technique is applied, trouble such as so-called "offset" tend to occur. The "offset" herein means the undesirable phenomenon that the toner image held on an image-receiving sheet are partially transferred onto the roller surface. This is a significant problem in the development of the heat roller fixing technique. In the heat roll fixing device generally used today, at least the surface layer of the roller which contacts with toner images is formed usually of silicon rubber or fluororesin, which have good mold releasing properties. But, in many systems, an oil having a mold releasing property such as silicone oil is applied on the silicon rubber or fluororesin surface for the purpose of preventing the offset on the surface and the fatigue of the roller surface. However, the oiling method has problems such that the addition of an oiling system complicates the fixing device and the oil vapor makes operators disagreeable. Accordingly, the approach to the prevention of offset by oiling is undesirable. It is rather desired to develop a toner which is fixable over a wide range of temperature and has a good anti-offset property. It is surely unsuitable in view of the design of a fixing system to demand excess mold-releasing properties of roll materials and lubricant oils the kinds of which are limited, in the development of the heat roll fixing technique. It is rather important for obtaining a compact, inexpensive fixing device to develop an offset-free toner while keeping the balance between the offset-free property and the development property.
The viscosity and the non-adhesiveness have so far been regarded as a point for the purpose of developing an offset free toner. It is important to design the toner composition so that the toner in the molten state will show a small viscosity change with temperature and have a suitable viscosity and further the toner will have low roll-adhesiveness. These, in a few words, are problems on thermal and physical properties of toners. However, the present inventors found out a phenomenon which cannot be explained merely from thermal and physical properties of toners, in the following experiments.
A toner was prepared by compounding 100 parts by weight of a styrene-butyl methacrylate copolymer, 10 parts by weight of low molecular weight polypropylene, and 6 parts by weight of a carbon black. This toner was mixed with a carrier iron powder to make up a negative-chargeable toner. A positive latent image was developed with this toner and transferred onto a plain paper. The unfixed image obtained is designated as N. The same toner (before mixing with the carrier) was mixed with a surface-coated carrier iron powder to make up a positive-chargeable toner. A negative latent image was developed with this toner and transferred onto a plain paper. The unfixed image obtained is designated as P. Then, fixing tests of the images N and P were conducted by using a fixing device which comprised a polytetrafluoroethylene-coated fixing roller containing a halogen lamp therein and a pressure roller coated with silicone rubber. The results were as follows:
TABLE 1 ______________________________________ Surface temp. (.degree.C.) of fixing roller 150 160 170 180 190 200 ______________________________________ Image N A o o o o o o B o o o o o o Image P A x .DELTA. .DELTA. o o o B x x x .DELTA. o o ______________________________________ Notes A: Fixability B: Resistance to offset o: good .DELTA.: slightly good x: poor
The same plain paper as used in the fixing tests had positive charge on passing through the fixing device, where the pair of rollers was negatively charged vs. the paper.
From this fact in view of the results above, the results of the fixing tests may be explained as follows: Since the paper will have positive charge at the time of fixing, electric force acts on the toner of the image to separate it from the paper and transfer to the roll when the toner is positively charged; thus the toner becomes difficult to attach on the paper and easy to result in the offset. On the contrary, the toner when charged negatively becomes easy to attach on the paper and difficult to result in the offset. In consequence, the image N is excellent in fixability and resistance to the offset, while the image P is inferior in fixability and resistance to the offset in spite of being formed of the same toner (except that the carrier is surface-treated). It should be noted that the image P is inferior in ressitance to the offset at lower temperatures (150.degree.-170.degree. C.). When the temperature of the fixing roller is high (190.degree.-200.degree. C.), the toner is sufficiently fused to adhere the paper and conceivably this eliminates the effect of electric force substantially.